ESTRUS AND LUTEAL FUNCTION IN SUCKLED BEEF-COWS THAT WERE ANESTROUS WHEN TREATED WITH AN INTRAVAGINAL DEVICE CONTAINING PROGESTERONE WITH OR WITHOUT A SUBSEQUENT INJECTION OF ESTRADIOL BENZOATE

The objectives in this study were to determine whether treatment with progesterone (P-4) via an intravaginal device would induce estrus and development of corpora lutea (CL) with typical life spans and whether treatment with estradiol benzoate (EB) following device removal would enhance the responses. At treatment initiation (d 0), suckled beef cow s (n = 362) that were anestrous received one of the following: 1) an i ntravaginal device containing P-4 for 7 d plus an injection of 1 mg of EB 24 to 30 h after device removal (P-4 + EB), 2) an intravaginal dev ice containing P-4 for 7 d (P-4), 3) a sham device for 7 d plus an inj ection of 1 mg of EB 24 to 30 h after device removal (EB), or 4) a sha m device for 7 d (control). Progesterone treatment increased the propo rtion of cows that formed CL with a typical lifespan (P < .001) and ex hibited behavioral estrus (P < .05). Treatment with EB in combination with P-4 increased (P < .001) the proportion of cows that exhibited es trus. Treatment with P-4 alone had no effect, but the combination of P -4 and EB increased (P < .01) the proportion of cows that formed short -lived or typical lifespan CL by the end of the experiment compared wi th untreated cows. In summary, treatment with P-4 increased the incide nce of estrus and the proportion of cows that formed CL with a typical lifespan. Treatment with P-4 resulted in resumption of luteal functio n in suckled beef cows that were anestrous, with most cows developing CL with a typical lifespan, whereas EB enhanced the expression of estr us.